For electronic systems, there historically have been two basic classes of power supplies: analog or linear power supplies and digital or switching mode power supplies. Each type of power supply had its advantages and limitations.
A linear power supply has very low noise levels, at the expense of having relatively low efficiency and being limited to output voltages that are lower than the input voltages. The efficiency is typically the ratio of the output voltage to the input voltage, with the output current equal to the input current at best. The primary application of a linear supply is to provide a highly regulated and stable output voltage with very low noise.
The other class of power supply, a switching power supply, on the other hand, can have a very high efficiency at the expense of having high levels of noise that are difficult or impossible to eliminate. The noise levels tend to vary inversely with the efficiency of the supply. For a very high efficiency supply, the switching device needs to spend the minimum time when the device is not saturated or open circuited. That is a minimum time when the switching device is switching. However, the impulse noise generated in the supply is due to the fast switching transients and extends to high frequencies, going to higher frequencies as the transition time decreases. Thus, the noise increases with efficiency.
A switching power supply can be configured in numerous ways including buck, boost, buck-boost, fly-back, and CUK. Since all typically use storage elements such as inductors and/or capacitors, the power supplies have the ability to transform voltages in ways that linear supplies cannot, enabling them to provide multiple simultaneous outputs at widely differing voltages and currents.
The two basic types of conventional power supplies each have their own short comings. Neither type can simultaneously provide voltage transformation and/or current transformation capabilities and low noise operation.